This happens just now when right-clicking on the thunderbird icon in the launcher, and at first it was unresponsive; the second time around I was able to quit thunderbird, but the whole desktop became unresponsive then compiz crashed...

Same here with but with Firefox. I try to close an instance of Nautilus via Menu/File/Quit and the desktop freezes and after few seconds recover from compiz crash with firefox down. I started a new instance of firefox and found that even the download windows of firefox was still active.

It looks to me like an incorrect pointer is being passed from NVIDIA's GL blob into XFindOnExtensionList(), so I concur with Sam's analysis that it's a driver bug, probably specific to -nvidia. This should be escalated to NVIDIA.

(This looks much like an NVIDIA-specific bug so far. If -fglrx has a similar crash, it could be coincidental, but should be investigated in its own right regardless, so please file a separate bug report.)